Device and Method for Determination of the Quantity of Nox Emitted by a Diesel Engine in a Motor Vehicle and Diagnostic and Engine Management System Comprising Such a Device

ABSTRACT

The invention relates to a device for determination of the quantity of NOx emitted by a diesel engine ( 10 ) in a motor vehicle with common-rail fuel supply means ( 12 ) for the cylinders thereof, of the type comprising pressure recording means ( 32 ) in at least one cylinder of the engine and means ( 50 ), for determining the mass fraction of oxygen in the mixture admitted into the cylinder. Said device comprises means ( 58, 68 ), for determining a temperature of the flame front on combustion of the mixture, means ( 52 ), for determining the mass of fuel burnt in the cylinder and means ( 70 ), for calculating the quantity of NOx emitted by the combustion of the mixture in the cylinder as a function of the recorded pressure, the mass fraction of oxygen in the mixture, the temperature of the flame front and the mass of fuel burnt.

The present invention relates to a device for determining the quantity of NOx emitted by a motor vehicle diesel engine associated with common rail means for supplying the engine cylinders with fuel, of the type comprising means for acquisition of data concerning the pressure in at least one cylinder of the engine and means for determining the mass fraction of oxygen in the intake mixture in the cylinder.

The invention also relates to systems for diagnosing and controlling the operation of the engine using such a device.

The amount of nitrous oxides, or NOx, emitted by a diesel engine is a piece of data important to its operation.

Indeed, the emission of NOx, which are pollutant molecules, must be minimised. To that end, the amount of fuel and the air flow injected into the cylinders are determined so that the formation of NOx during combustion of the mixture in the cylinders is minimised.

The engine is also generally associated with depollution means arranged in its exhaust line such as, for example, a NOx trap, and the operation of the engine is then controlled in order to optimise the operation of the depollution means. The engine may thus be controlled according to several modes of operation by modifying the amounts of fuel and air injected into the cylinders. For example, the engine may operate in rich mode in order to regenerate the NOx trap.

Poor adjustment of the engine, due, for example to aging of the injectors and/or the cylinders, has the effect of increasing the emission of NOx. Thus, the quantity of NOx emitted by a diesel engine is representative of the operating state of the engine.

Accurate knowledge of the amount of NOx emitted by the engine makes it possible to optimise both the operation of the engine and the amount of pollutant expelled into the atmosphere by the vehicle.

Devices for determining the amount of NOx emitted by a motor vehicle diesel engine associated with common-rail means for supplying the engine cylinders with fuel use engine adjustment values to determine the amount of NOx emitted, such as, for example, injection maps and/or EGR maps of air flow if the engine is associated with an exhaust gas recirculation (EGR) loop.

However, such systems are not based on the actual features of the operation of the engine but on adjustment values which are predetermined at the factory.

Moreover, the features of the engine change over time because of aging of its components such as, for example, the injectors and the cylinders. Thus, where there are major drifts in these features, the amount of NOx determined may be highly erroneous.

Other systems for determining the amount of NOx emitted by a diesel engine determine the mean temperature of the burning mixture in the cylinders in order to deduce from it an amount of NOx at equilibrium and thereafter the mass of NOx emitted by the engine per engine cycle.

However, under certain conditions, the results returned by such systems are of relatively low accuracy and such systems do not allow the amount of NOx to be calculated at each moment of the combustion phase of the engine cylinders.

The aim of the present invention is to solve the aforementioned problem by proposing a device for determining the amount of NOx emitted by a diesel engine which is accurate, requires little calculation time and which determines in real time the amount of NOx emitted by the engine.

To that end, the invention relates to a device for determining the amount of NOx emitted by a motor vehicle diesel engine associated with common rail means for supplying the engine cylinders with fuel, of the type comprising means for acquisition of data concerning the pressure in at least one cylinder of the engine and means for determining the mass fraction of oxygen in the intake mixture in the cylinder, characterised in that it includes

-   -   means for determining a flame front temperature during         combustion of the intake mixture in the cylinder;     -   means for determining the mass of fuel burned in the cylinder;     -   means for calculating the amount of NOx emitted by combustion of         the mixture in the cylinder as a function of the detected         pressure and the values determined for the mass fraction of         oxygen in the mixture, the flame front temperature and the mass         of fuel burned.

According to particular embodiments, the above-mentioned device includes at least one of the following features:

-   -   the means for determining the mass of fuel burned in the         cylinder include means for determining the amount of         instantaneous heat released during combustion of the intake         mixture in the cylinder and means for determining the         instantaneous mass of fuel burned in the cylinders as a function         of this latter and the calorific potential of the fuel injected         into the cylinder;     -   the means for determining the amount of instantaneous heat         released during combustion of the mixture are adapted to         determine this amount as a function of the crank angle of the         cylinder and the pressure within the cylinder, using the first         law of thermodynamics;     -   the means for determining the flame front temperature include         means for determining the temperature of the unburned intake         mixture during combustion of that mixture and means for         determining the flame front temperature in the cylinder as a         function of that temperature of the unburned intake mixture;     -   the means for determining the temperature of the unburned intake         mixture during combustion of that mixture are adapted to do so         using a thermodynamic model of isentropic compression according         to the equation:         $T_{nb} = {T_{nb}^{0}\left( \frac{P^{nb}}{P^{0}} \right)}^{\frac{k}{k - 1}}$         where T_(nb) and P^(nb) are respectively the temperature of the         unburned intake mixture and the corresponding pressure in the         cylinder during combustion of the mixture, T_(nb) ⁰ and P⁰ are         respectively a reference temperature and reference pressure of         the intake mixture in the cylinder at a predetermined moment         before the start of combustion of the intake mixture, and k is a         predetermined polytropic coefficient;     -   it includes means for determining the amount of instantaneous         heat released during combustion of the intake mixture in the         cylinder and means for determining the moment when combustion of         the intake mixture starts capable of comparing the determined         instantaneous amount of heat with a predetermined threshold         value and of determining the moment when combustion starts when         the determined instantaneous amount of heat is greater than the         threshold value;     -   the means for determining the temperature of the intake mixture         during combustion of that mixture includes:     -   means for determining the number of moles of the intake mixture         in the cylinder; and     -   means for determining the reference temperature T_(nb) ⁰ as a         function of the number of moles of the intake mixture and the         pressure P⁰ in the cylinders at the predetermined moment before         combustion begins, based on a thermodynamic model of the intake         mixture;     -   the means for determining the reference temperature T_(nb) ⁰ is         adapted to do so according to the equation:         $T_{nb}^{0} = \frac{P^{0} \times V^{0}}{n \times R}$     -   the means for determining the flame front temperature are         adapted to determine a theoretical adiabatic temperature of the         flame front;     -   the means for determining the adiabatic temperature of the flame         front are adapted to determine that temperature as a function of         the temperature of the unburned intake mixture during combustion         of the intake mixture and of the mass fraction of oxygen in the         intake mixture, using a thermodynamic model of conservation of         the enthalpy of the reagents and the products of combustion of         the intake mixture in the cylinder;     -   the thermodynamic model of conservation of enthalpy is a         polynomial model of the first or second order;     -   the polynomial model is a model according to the equation:         T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂         where T_(ad) is the adiabatic temperature of the flame front,         XO₂ is the mass fraction of oxygen in the mixture, and c₁, c₂         and c₃ are predetermined coefficients;     -   the polynomial model is a model according to the equation:         T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P         where T_(ad) is the adiabatic temperature of the flame front,         XO₂ is the mass fraction of oxygen in the mixture, P is the         pressure in the cylinder, and c₁, c₂, c₃, c₄ are predetermined         coefficients;     -   the polynomial model is a model according to the equation:         T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P+c ₅ ×XO₂ ²         where T_(ad) is the adiabatic temperature of the flame front,         XO₂ is the mass fraction of oxygen in the mixture, P is the         pressure in the cylinder, and c₁, c₂, c₃, c₄ and c₅ are         predetermined coefficients;     -   the means for determining the amount of NOx emitted by the         combustion of the intake mixture in the cylinder are adapted to         do so using a chemical model of production of NOx during         combustion of the mixture in the cylinder; and     -   the chemical model is a model according to the equation:         $Q_{NOx} = {\frac{{\ln(P)} + 1}{b \times X\quad O_{2}} \times {\exp\left( \frac{T_{ad} - c}{d \times X\quad O_{2}} \right)} \times \frac{M\quad C\quad B}{M\quad C\quad I}}$         where Q_(NOx) is the instantaneous amount of NOx emitted by the         engine, P is the pressure in the cylinder, T_(ad) is the         adiabatic temperature of the flame front in the cylinder, XO₂ is         the mass fraction of oxygen in the intake mixture in the         cylinder, MCB is the instantaneous mass of fuel burned in the         cylinder, MCI is the mass of fuel injected into the cylinder,         and b, c and d are predetermined parameters.

The object of the invention is also a method for determining the amount of NOx emitted by a motor vehicle diesel engine comprising common rail means for supplying the engine cylinders with fuel, of the type comprising a step of acquisition of data concerning pressure in at least one cylinder of the engine, and a step of determination of the mass fraction of oxygen in the intake mixture in the cylinder, characterised in that it includes a step of:

-   -   determination of the flame front temperature during combustion         of the intake mixture in the cylinder;     -   determination of the mass of fuel burned in the cylinder; and of     -   calculation of the amount of NOx emitted by combustion of the         mixture in the cylinder as a function of the acquired pressure         and of the values determined for the mass fraction of oxygen in         the mixture, the flame front temperature and the mass of fuel         burned.

The object of the invention is also a system for diagnosing the malfunctioning of a motor vehicle diesel engine, characterised in that it includes a device of the aforementioned type, means for comparing the amount of NOx emitted with a predetermined threshold and means for triggering an alarm when the amount of NOx exceeds this threshold.

The object of the invention is also a system for controlling the operation of a motor vehicle diesel engine associated with means for depollution of NOx arranged in an exhaust line of the engine, characterised in that it includes a device of the aforementioned type, means for calculating the amount of NOx stored in the depollution means as a function of the amount of NOx determined by the device and means for controlling the operation of the engine according to the amount of NOx stored, in order to manage the operation of the depollution means.

The object of the invention is also a system for controlling the operation of a motor vehicle diesel engine, characterised in that it includes a device of the aforementioned type and adjustment means adapted to adjust the operation of the supply means according to the determined amount of NOx emitted, in order to correct drifts in the operation thereof.

According to another feature, this system is characterised in that the engine is associated with means for recirculating part of the exhaust gases at the engine intake, and in that the adjustment means are also adapted to adjust the operation of the recirculation means according to the determined quantity of NOx emitted, in order to correct drifts in the operation of the supply means and/or the recirculation means.

A clear understanding of the present invention will be facilitated by the following description, given solely by way of example and described in reference to the appended drawings, in which:

FIG. 1 is a schematic view of a diesel engine motor vehicle propulsion unit associated with a device according to the invention.

FIG. 2 is a more detailed schematic view of the device according to the invention;

FIG. 3 is a flowchart of the operation of the device according to the invention; and

FIG. 4 is a graph of the results returned by the device according to the invention during a series of tests.

In FIG. 1, a motor vehicle diesel engine 10 is associated with common rail means 12 for supplying the engine cylinders with fuel, for example, comprising a common supply rail delivering fuel at high pressure to controlled injectors capable of injecting fuel into the cylinders of the engine 10 in the form of multiple injections, for example.

The engine 10 is also associated with a loop 14 for recirculation of part of the exhaust gases, or EGR, at the engine intake. The recirculation loop 14 includes a bypass line 16 of an exhaust line 18 of the engine 10. This bypass line 16 is capable of taking exhaust gases leaving the engine 10 and delivering them to means 20 taking in air/exhaust gas mixture at the intake of the engine 10. These intake means 20 also receive air from an air inlet 22 and deliver an air/exhaust gas mixture to the engine 10.

For the purposes of processing the emissions of pollutants from the engine 10, and in particular the emission of nitrous oxides, or NOx, depollution means 24 are arranged in the exhaust line 18. The depollution means 24 include for example a NOx trap adapted to store NOx and release them in a non-polluting form for expulsion into the atmosphere.

Normally, the operation of the engine and the components which have just been described is controlled by a unit 30 for controlling the operation of the engine.

The unit 30 is connected to means 32 for detecting (i) the pressure in each cylinder of the engine, comprising, for example, a piezoelectric deformation sensor arranged in the head of the cylinder and adapted to measure the pressure in the combustion chamber of the cylinder, (ii) the engine speed, comprising for example a speed sensor, (iii) the engine torque desired by the driver of the vehicle, comprising for example a sensor for sensing the position of the accelerator pedal of the vehicle, and (iv) the engine timing angle, comprising for example a Hall effect sensor arranged on the engine drive shaft.

The unit 30 is also connected to means 34 for detecting the flow rate of air entering the engine, for example a flowmeter arranged in the air inlet 22 of the intake means 20.

The unit 30 is adapted to determine injection settings for the supply means 12, in particular a pilot injection setting and a main injection setting for each cylinder and for each engine cycle, as a function of the engine speed, the torque and the crank angle of the cylinder, this latter being determined by the unit 30 according to the engine timing angle detected.

The unit 30 also determines, for the engine cycle, an EGR air flow rate setting for the intake means 20 according to the engine speed, torque and crank angle of the cylinder.

The unit 30 is also adapted to implement a strategy of managing the operation of the depollution means 24 by controlling the phasing and/or the amount of fuel injected into the cylinders in order to manage the storage/release states of the depollution means 24.

The engine 10 is associated with a device according to the invention for determining the amount of NOx emitted by the engine. This device determines such an amount on the basis of the amount of intake mixture burned in the combustion chamber of each cylinder when a flame front propagates within it, the intake mixture in the cylinder being defined as the sum of the amounts of fresh air, exhaust gas and fuel taken into the cylinder.

In the example illustrated in FIG. 1, this device is implemented by a sub-unit 36 of the unit 30. In another variant, the device may also be implemented by a dedicated data processing unit.

A description will now be given, with reference to FIGS. 2 and 3, of the arrangement and operation of the device for determining the amount of NOx emitted by the engine 10.

The amount of NOx emitted by the engine is determined in accordance with a chemical model of the production of NOx during combustion of the intake mixture in an engine cylinder. This model uses as a variable the mass fraction of oxygen XO₂ in the intake mixture in the cylinder, the instantaneous mass MCB of fuel burned in the cylinder, the pressure P in the cylinder and a theoretical temperature T_(ad) of the flame front propagating in the combustion chamber of the cylinder, and preferably, a theoretical adiabatic temperature of the flame front, as will be explained below in more detail.

The device for determining the amount of NOx emitted by the engine 10 includes means 50 for determining the mass fraction of oxygen XO₂ in the intake mixture to be burned in the cylinder during an engine cycle. These means 50 receive as input the detected air flow rate DA and the rate TEGR of recycled exhaust gas entering the engine.

The rate TEGR of recycled exhaust gas is determined by the unit 30 as a function of the detected air flow rate DA and the operating point of the engine, for example using a predetermined map stored in memory in the unit 30.

The means 50 also receive the total amount MCI of fuel injected into the cylinder for the engine cycle and are adapted to determine the richness of the intake mixture according to that amount, as is known in the art. This amount MCI is determined by the unit 30 according to the injection settings delivered to the supply means 12, for example by adding together the quantities of fuel injected into the cylinder for the engine cycle.

The means 50 for determining the mass fraction of oxygen XO₂ in the mixture then determine this fraction as a function of the richness of the intake mixture and the determined TEGR rate using as its basis a combustion balance of the intake mixture, the mass fraction of oxygen XO₂ admitted being usually directly proportional to the richness and the EGR rate, as is known in the state of the art.

The device according to the invention also includes means 52 for determining the instantaneous amount MCB of fuel burned in the cylinder during the engine cycle.

To that end, the means 52 include means 54 for determining the instantaneous amount of heat released by combustion of the mixture in the cylinder during the combustion phase of the cycle of the cylinder. This determination is performed as a function of the detected pressure P in the combustion chamber of the cylinder and the crank angle α of the cylinder, using the first law of thermodynamics, according to the equation: $\begin{matrix} {\frac{\mathbb{d}Q}{\mathbb{d}\alpha} = {\frac{1}{k - 1} \times \left( {{V \times \frac{\mathbb{d}P}{\mathbb{d}\alpha}} - {k \times P \times \frac{\mathbb{d}V}{\mathbb{d}\alpha}}} \right)}} & (1) \end{matrix}$ where dα is a predetermined variation of the crank angle α of the cylinder, dQ is the instantaneous amount of heat released by combustion of the mixture during the variation dα in the crank angle, V and P are respectively the volume of the combustion chamber and the pressure within it at the moment when the variation dα of the crank angle begins, dV and dP are respectively the variation in the volume of the combustion chamber and the variation in the pressure within it corresponding to the variation dα in the crank angle, and k is a predetermined polytropic coefficient.

This determined amount of heat dQ is delivered to means 56 for determining the corresponding amount of fuel burned. The means 56 are capable of determining this amount of fuel by dividing the amount of heat dQ by the value of the mass energy content of the fuel used in the engine, or NCV for net calorific value (in J/kg). The NCV value is for example mapped in the means 56.

The device according to the invention also include means 58 for determining the temperature T_(nb) of the unburned intake mixture at a moment after the start of combustion of the mixture in the cylinder. This temperature T_(nb) of the unburned intake mixture is calculated by making a hypothesis of isentropic compression of the unburned intake mixture from a moment which precedes the start of combustion. This temperature T_(nb) of the unburned intake mixture is then used to determine the theoretical adiabatic temperature T_(ad) of the flame front propagating inside the combustion chamber of the cylinder, as will be explained in more detail below.

The means 58 for determining the temperature T_(nb) include means 60 for determining the number of moles n of the intake mixture present in the combustion chamber of the cylinder before the start of combustion as a function of the detected air flow rate DA, the rate of recirculated exhaust gas TEGR entering the engine and the total amount of fuel MCI injected into the cylinder.

The number n of moles is then delivered to means 62 for determining the temperature T_(nb) ⁰ of the intake mixture at a predetermined moment before the start of combustion in the cylinder, for example corresponding to a crank angle α included within the range of crank angles [−60°; −20°] before top dead centre (TDC) of the cylinder cycle.

Detection of the moment when combustion of the mixture starts is performed by means 64 for comparing the instantaneous amount of heat dQ released by combustion of the intake mixture, determined by the means 54, at a predetermined threshold.

When the amount of heat dQ reaches this threshold value, the start of combustion of the intake mixture is detected and calculation of the temperature T_(nb) of the unburned intake mixture during combustion is triggered.

The means 62 then determines the temperature T_(nb) by considering the intake mixture to be a perfect gas according to the equation: $\begin{matrix} {T_{nb}^{0} = \frac{P^{0} \times V^{0}}{n \times R}} & (2) \end{matrix}$ where V⁰ and P⁰ are the volume of the combustion chamber and the pressure within it at the predetermined moment before the start of combustion of the intake mixture, and R is the perfect gases constant.

The values of V⁰ and of P⁰ are for example stored in memory in the means 62 after the last acquisition of data concerning the pressure P in the cylinder for the crank angle α⁰ included within the range of crank angles [−60°;−20°] before top dead centre of the cylinder cycle, the crank angle α⁰ corresponding to the volume V⁰ of the combustion chamber of the cylinder.

The temperature T_(nb) ⁰ and the pressure before the start of combustion P⁰ are delivered as the reference temperature and reference pressure to means 66 for determining the temperature T_(nb) of the unburned intake mixture during combustion, that is, during propagation of the flame front in the combustion chamber of the cylinder. The means 66 determine this temperature using a thermodynamic model of isentropic compression in the compression phase of the cylinder cycle according to the equation: $\begin{matrix} {T_{nb} = {T_{nb}^{0}\left( \frac{P^{nb}}{P^{0}} \right)}^{\frac{k}{k - 1}}} & (3) \end{matrix}$

The means 66 determine the temperature T_(nb) continuously during a period of time corresponding to the combustion of the intake mixture in the cylinder. This period corresponds for example to the range of crank angles [0; 120°] after top dead centre if the engine load is partial or the range [−15 ; 120°] relative to TDC if the engine load is approximately at maximum.

The determined temperature T_(nb) is delivered to means 68 for determining the adiabatic temperature T_(ad) of the flame front during combustion of the intake mixture in the combustion chamber of the cylinder.

These means determine the temperature T_(ad) using a thermodynamic model of conservation of the enthalpy of the reagents and of the products of combustion of the mixture according to the equation: H _(initial)(P,T _(nb) XO₂)=H _(final)(P,T _(ad) ,XO₂)  (4) where H_(initial) is the enthalpy of the intake mixture before the moment when combustion of the latter starts and H_(final) is the intake enthalpy of the exhaust gases produced by combustion of the intake mixture by the flame front.

Advantageously, this model of conservation of enthalpy is approximated by a polynomial model, the adiabatic temperature T_(ad) of the flame front being determined by the means 68 according to the equation: T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂  (5) where c₁, c₂ and c₃ are predetermined coefficients.

The correlation between the adiabatic temperature determined according to equation (5) and an adiabatic temperature determined using a complex model of the same based on equation (4) has a correlation coefficient R² approximately equal to 99.43%.

In another embodiment, the means 68 also receive as input the pressure P measured in the combustion chamber of the cylinder and determine the adiabatic temperature of the flame front according to the equation: T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P  (6) where c₁, c₂, c₃, c₄ are predetermined coefficients.

The introduction of the pressure P in the cylinder into equation (6) gives a coefficient R² approximately equal to 99.5%.

In another embodiment, the means 68 also receive as input the pressure P measured in the combustion chamber of the cylinder and determines the adiabatic temperature of the flame front according to the equation: T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P+c ₅ ×XO₂ ²  (7) where c₁, c₂, c₃, c₄ et c₅ are predetermined coefficients.

The introduction of the square of the mass fraction XO₂ into equation (7) gives a coefficient R² approximately equal to 99.97%.

Thus, the means 68 determine the adiabatic temperature T_(ad) of the flame front in a manner which is simple and requires little calculation time, whilst determining that temperature reliably.

Finally, the device according to the invention includes means 70 for calculating the amount of instantaneous NOx emitted by combustion of the intake mixture in the cylinder as a function of the pressure P in the cylinder, the adiabatic temperature T_(ad) of the flame front, the mass fraction of oxygen XO₂ in the mixture and the instantaneous mass MCB of fuel burned. This calculation is performed using a predetermined chemical model of production of NOx in the cylinder, for example according to the equation: $\begin{matrix} {Q_{NOx} = {\frac{{\ln(P)} + 1}{b \times X\quad O_{2}} \times {\exp\left( \frac{T_{ad} - c}{d \times X\quad O_{2}} \right)} \times \frac{M\quad C\quad B}{M\quad C\quad I}}} & (8) \end{matrix}$ where Q_(NOx) is the instantaneous amount of NOx emitted by combustion of the intake mixture in the cylinder in grams per kilogram of fuel injected into the cylinder for each crank degree, and b, c and d are predetermined parameters.

Because of the time lag between the combustion phases in the cylinders, which never take place simultaneously, the instantaneous amount Q_(NOx) of NOx produced during combustion of the mixture in the cylinder is thus approximately equal to that emitted by the engine 10.

FIG. 3 is a flowchart of the operation of the device just described for determining the amount of NOx emitted by the engine.

After starting the vehicle, the operation consists at 50 of selecting the reference i of the cylinder within which the next combustion of mixture will take place.

Then, at 52, the total mass of fuel MCI, the air flow rate DA and the rate of recirculated exhaust gas TEGR injected into this cylinder i are determined.

A subsequent step 54 then consists of determining, as a function of the values determined at 52, the richness of the intake mixture and then the mass fraction of oxygen XO₂ in the intake mixture in the cylinder i.

The operation of the device according to the invention then consists at 56 of determining the instantaneous amount of heat dQ released by combustion of the intake mixture in the cylinder i according to equation (1) and of comparing this, at 58, with the threshold value for detecting the moment when combustion of the mixture starts.

As long as this moment is not detected, that is, as long as the amount dQ is lower than the detection threshold, the method 58 loops back to step 56. If the moment when combustion starts is detected at 58, a subsequent step 60 of the operation is a step to determine the temperature T_(nb) ⁰ of the mixture at the predetermined moment before the start of combustion thereof according to equation (2).

Step 60 is then followed by a step 62 to determine the temperature T_(nb) of the unburned intake mixture at a moment after the start of combustion according to equation (3). Step 62 continues by determining the adiabatic temperature T_(ad) of the flame front according to equation (5) as a function of the temperature T_(nb) of the unburned intake mixture, the mass fraction of oxygen XO₂ in the mixture, and the pressure P of the cylinder i if equation (6) or equation (7) is used.

The instantaneous mass MCB of fuel burned in the cylinder i is then determined at 64 as a function of the amount of heat determined previously, as has been described above.

The operation then continues via a step 66 to determine the instantaneous amount QNOx of NOx emitted by combustion of the mixture in the cylinder i according to equation (8).

Following the determination of the amount Q_(NOx), a test is performed at 68 to find out whether combustion of the mixture in the cylinder i has finished, for example by testing whether the determined instantaneous amount of heat dQ is below a second predetermined threshold value.

If the result of this test is positive, step 68 then loops back to step 50 to select a new cylinder i.

If the result of this test is negative, a new instantaneous amount of heat dQ is determined at 70.

Step 70 then loops back to step 62 to determine a new instantaneous amount Q_(NOx) of NOx emitted by combustion of the mixture in the cylinder i at a moment following combustion, by implementing steps 62, 64 and 66.

The device according to the invention implements a determination algorithm requiring a small number of calculations, whilst allowing the amount of NOx emitted by the engine to be determined in real time and instantaneously, that is, at each moment in the combustion phase of the cylinder.

Other embodiments of the device according to the invention are possible.

For example, as a variant, the device includes a data acquisition chain for the pressure in a single cylinder of the engine and the device is capable of determining the amount of NOx emitted by combustion of the intake mixture in this cylinder and of multiplying the determined amount of NOx by the number of cylinders in the engine in order to obtain the total amount of NOx emitted by the engine.

As a variant, the device includes a data acquisition chain for the pressure in any number n of engine cylinders, and is capable of determining the amount of NOx emitted by these systems and of multiplying this latter by $\frac{N}{n},$ where N is the number of engine cylinders, in order to obtain the total amount of NOx emitted by the engine.

FIG. 4 shows the accuracy of the determination of the amount of NOx emitted by the engine implemented by the device according to the invention. The x-axis shows, for different points of operation of a diesel test engine, the amount of NOx determined using a complex physical model of NOx production and the y-axis shows the corresponding amounts obtained by the device according to the invention.

The device according to the invention thus enables a high degree of accuracy to be obtained, in a simple manner, for a large range of operation of the engine.

Thus, it is possible to use such a device in more complex systems for diagnosing and/or controlling the operation of the diesel engine 10 using data on the amount of NOx emitted by the engine.

A first system is a system for diagnosing malfunctioning of the engine 10. Indeed, if the emission of NOx is abnormally high, a malfunction of the engine 10 may be diagnosed.

To that end, the diagnostic system includes a device according to the invention which delivers the instantaneous amount of NOx emitted by the engine to means for comparing this amount with a predetermined threshold. Means for triggering an alarm receive the result of this comparison and trigger an alarm, for example, activation of an indicator light on the instrument panel of the vehicle, when the determined amount of NOx exceeds this threshold.

It is also possible to envisage a system controlling the operation of the diesel engine 10 in order to manage of the states of storage and release of the depollution means 24 on the basis of the amount of NOx emitted by the engine.

Such a system includes, for example, a device according to the invention for determining the amount of NOx emitted by the engine and delivering that amount to means for calculating the amount of NOx stored in the depollution means 24 as a function of that amount.

The determined amount of stored NOx is then delivered to means for comparing that amount with first and second predetermined thresholds. Means for triggering the regeneration of the depollution means 24 receive the result of this comparison and trigger the operation of the engine 10 in the mode for regeneration of the depollution means 24 when the amount of NOx stored in the latter is above the first threshold, and deactivate such a mode of operation of the engine 10 when the amount of stored NOx is below the second threshold.

The regeneration of the depollution means is thus triggered according to an item of data which remains pertinent throughout the life of the vehicle. The operation of the engine associated with the management of the depollution means 24 is thus optimised.

It is also possible to envisage a system for controlling the operation of the engine 10 comprising the device according to the invention and means for adjusting the operation of the means 12 for supplying the engine 10. The adjustment means are adapted to adjust the operation of the supply means 12 according to the amount of NOx emitted determined by the device in order to correct drifts in the operation of the supply means. For example, the means of adjusting the supply means 12 are capable of adjusting the phasing and/or the quantities of fuel injected into the cylinders in order to minimise the emission of NOx by the engine 10.

The adjustment means may also be adapted to adjust the operation of the recirculation loop 14 according to the amount of NOx emitted by the engine 10 in order to correct drifts in the operation of the supply means 12 and/or the recirculation loop 14, in order likewise to minimise the emission of NOx. 

1. Device for determining the amount of NOx emitted by a motor vehicle diesel engine associated with common rail means for supplying the engine cylinders with fuel, of the type comprising means for acquisition of data concerning pressure in at least one cylinder of the engine and means for determining the mass fraction of oxygen in the intake mixture in the cylinder, wherein said device includes means for determining a flame front temperature during combustion of the intake mixture in the cylinder; means for determining the mass of fuel burned in the cylinder; and means for calculating the amount of NOx emitted by combustion of the mixture in the cylinder as a function of the detected pressure and the values determined for the mass fraction of oxygen in the mixture, the flame front temperature and the mass of fuel burned.
 2. Device according to claim 1, wherein the means for determining the mass of fuel burned in the cylinder include means for determining the instantaneous amount of heat released during combustion of the intake mixture in the cylinder and means for determining the instantaneous mass of fuel burned in the cylinders as a function of this latter and of the calorific potential of the fuel injected into the cylinder.
 3. Device according to claim 2, wherein the means for determining the instantaneous amount of heat released during combustion of the mixture are adapted to determine this amount as a function of the crank angle of the cylinder and of the pressure within the cylinder, using the first law of thermodynamics.
 4. Device according to claim 1 wherein the means for determining the flame front temperature include means for determining the temperature of the unburned intake mixture during combustion of that mixture and means for determining the flame front temperature in the cylinder as a function of that temperature of the unburned intake mixture.
 5. Device according to claim 4, wherein the means for determining the temperature of the unburned intake mixture during combustion of that mixture are adapted to do so using a thermodynamic model of isentropic compression according to the equation: $T_{nb} = {T_{nb}^{0}\left( \frac{P^{nb}}{P^{0}} \right)}^{\frac{k}{k - 1}}$ where T_(nb) and P^(nb) are respectively the temperature of the unburned intake mixture and the corresponding pressure in the cylinder during combustion of the mixture, T_(nb) ⁰ and P⁰ are respectively a reference temperature and reference pressure of the intake mixture in the cylinder at a predetermined moment before the start of combustion of the intake mixture, and k is a predetermined polytropic coefficient.
 6. Device according to claim 4, which includes means for determining the instantaneous amount of heat released during combustion of the intake mixture in the cylinder and means for determining the moment when combustion of the intake mixture starts capable of comparing the determined instantaneous amount of heat with a predetermined threshold value and of determining the moment when combustion starts when the determined instantaneous amount of heat exceeds the threshold value.
 7. Device according to claim 5 wherein the means for determining the temperature of the intake mixture during combustion of the latter include: means for determining the number of moles of the intake mixture in the cylinder; and means for determining the reference temperature T_(nb) ⁰ as a function of the number of moles of the intake mixture and the pressure P⁰ in the cylinders at the predetermined moment before combustion starts, based on a thermodynamic model of the intake mixture.
 8. Device according to wherein the means for determining the reference temperature T_(nb) are adapted to do so according to the equation: $T_{nb}^{0} = \frac{P^{0} \times V^{0}}{n \times R}$
 9. Device according to claim 1 wherein the means for determining the flame front temperature are adapted to determine a theoretical adiabatic temperature of the flame front.
 10. Device according to claim 4, wherein the means for determining the flame front temperature are adapted to determine a theoretical adiabatic temperature of the flame front, and the means for determining the adiabatic temperature of the flame front are adapted to determine it as a function of the temperature of the unburned intake mixture during combustion of that mixture and the mass fraction of oxygen in the intake mixture, based on a thermodynamic model of conservation of the enthalpy of the reagents and of the products of combustion of the intake mixture in the cylinder.
 11. Device according to claim 10, wherein the thermodynamic model of conservation of enthalpy is a polynomial model of the first or second order.
 12. Device according to claim 11, wherein the polynomial model is a model according to the equation: T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ where T_(ad) is the adiabatic temperature of the flame front, XO₂ is the mass fraction of oxygen in the mixture, and c₁, c₂ and c₃ are predetermined coefficients.
 13. Device according to claim 11, wherein the polynomial model is a model according to the equation: T_(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P where T_(ad) is the adiabatic temperature of the flame front, XO₂ is the mass fraction of oxygen in the mixture, P is the pressure in the cylinder, and c₁, c₂, c₃, c₄ are predetermined coefficients.
 14. Device according to claim 11, wherein the polynomial model is a model according to the equation: T _(ad) =c ₁ +c ₂ ×T _(nb) +c ₃ ×XO₂ +c ₄ ×P+c ₅ ×XO₂ ² where T_(ad) is the adiabatic temperature of the flame front, XO₂ is the mass fraction of oxygen in the mixture, P is the pressure in the cylinder, and c₁, c₂, c₃, c₄ and c₅ are predetermined coefficients.
 15. Device according to claim 1, wherein the means for determining the amount of NOx emitted by combustion of the intake mixture in the cylinder are adapted to do so using a chemical model of the production of NOx during combustion of the mixture in the cylinder.
 16. Device according to claim 15, wherein the means for determining the flame front temperature are adapted to determine a theoretical adiabatic temperature of the flame front, and the chemical model is a model according to the equation: $Q_{NOx} = {\frac{{\ln(P)} + 1}{b \times {XO}_{2}} \times {\exp\left( \frac{T_{ad} - c}{d \times {XO}_{2}} \right)} \times \frac{MCB}{MCI}}$ where Q_(NOx) is the instantaneous amount of NOx emitted by the engine, P is the pressure in the cylinder, T_(ad) is the adiabatic temperature of the flame front in the cylinder, XO₂ is the mass fraction of oxygen in the intake mixture in the cylinder, MCB is the instantaneous mass of fuel burned in the cylinder, MCI is the mass of fuel injected into the cylinder, and b, c and d are predetermined parameters.
 17. System for diagnosing malfunctioning of a motor vehicle diesel engine, which includes a device according to claim 1, means for comparing the amount of NOx emitted with a predetermined threshold and means for triggering an alarm when the amount of NOx exceeds this threshold.
 18. System for controlling the operation of a motor vehicle diesel engine associated with means for depollution of NOx arranged in an exhaust line of the engine, which includes a device according to claim 1, means for calculating the amount of NOx stored in the depollution means as a function of the amount of NOx determined by the device and means for controlling the operation of the engine, according to the amount of NOx stored, in order to manage the operation of the depollution means.
 19. System for controlling the operation of a motor vehicle diesel engine, which includes a device according to claim 1 and adjustment means adapted to adjust the operation of the supply means according to the determined amount of NOx emitted, in order to correct drifts in the operation of the supply means.
 20. System according to claim 19, wherein the engine is associated with means for recirculating a part of the exhaust gases into the engine, and in that the adjustment means are also adapted to adjust the operation of the recirculation means according to the determined amount of NOx emitted, in order to correct drifts in the operation of the supply means and/or the recirculation means.
 21. Method for determining the amount of NOx emitted by a motor vehicle diesel engine comprising common rail means for supplying fuel to the engine cylinders, of the type comprising a step for acquisition of data concerning pressure in at least one cylinder of the engine and a step for determining the mass fraction of oxygen in the intake mixture in the cylinder, characterised in that it includes a step for: determining the temperature of the flame front during combustion of the intake mixture in the cylinder; determining the mass of fuel burned in the cylinder; and calculating the amount of NOx emitted by combustion of the mixture in the cylinder as a function of the detected pressure, and the values determined for the mass fraction of oxygen in the mixture, the flame front temperature and the mass of fuel burned. 